Optical coherence tomographic imaging apparatus and method for controlling the same

ABSTRACT

An optical coherence tomographic imaging apparatus includes a movement amount acquisition unit configured to acquire the amount of subject&#39;s eye movement based on a plurality of images of the subject&#39;s eye acquired at different times, a determination unit configured to determine whether the amount of subject&#39;s eye movement before a scan by the scanning unit exceeds a threshold value, and a control unit configured to, in a case the amount of subject&#39;s eye movement before the scan is equal to or smaller than the threshold value, control the scanning unit to perform scanning position correction between a scan and the next scan based on the amount of movement.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical coherence tomographicimaging apparatus for capturing a tomographic image of a subject's eyethrough optical coherence, and a method for controlling the opticalcoherence tomographic imaging apparatus.

2. Description of the Related Art

A presently known optical coherence tomographic imaging apparatus basedon optical coherence tomography (OCT) utilizes multi-wavelength lightwave interference. For example, the optical coherence tomographicimaging apparatus is used to acquire internal organ information with anendoscope and retina information in an ophthalmologic apparatus, andapplied to increasing number of fields in the human body. An opticalcoherence tomographic imaging apparatus applied to the human eye isbecoming essential for specialized retina clinic, as an ophthalmologicapparatus.

Such an optical coherence tomographic imaging apparatus is capable ofirradiating a sample with measuring light which is low-coherent light,and measuring backward scattering light from the sample by using aninterferometer. In a case one point on the sample is irradiated with themeasuring light, image information in a depth direction at the one pointon the sample can be acquired. Further, by performing measurement whilescanning the sample with the measuring light, a tomographic image of thesample can be acquired. In a case the optical coherence tomographicimaging apparatus is applied to the fundus, the fundus of the subject'seye is scanned with the measuring light to capture a high-resolutiontomographic image of the fundus of the subject's eye. Therefore, theoptical coherence tomographic imaging apparatus is widely used inophthalmology diagnosis of the retina.

Generally, the optical coherence tomographic imaging apparatus uses amethod for capturing a plurality of tomographic images by repetitivelyscanning a measurement target (fundus), in a horizontal or verticaldirection. Thus, the optical coherence tomographic imaging apparatusscans an identical region on the fundus a plurality of times to capturea plurality of tomographic images of the identical region, and performsaveraging processing on the captured tomographic images to acquire ahigh-definition tomographic image of the fundus. Further, by scanningthe fundus a plurality of times while moving the scanning position inparallel, a three-dimensional image of the fundus can be acquired. In acase scanning the fundus a plurality of times in this way, however,since it takes a certain amount of time to complete image capturing, theeye may move during image capturing.

Japanese Patent Application Laid-Open No. 2008-29467 discusses anophthalmologic imaging apparatus having a tracking function.Specifically, the ophthalmologic imaging apparatus successively capturesa plurality of front images of the subject's eye, detects the subject'seye movement by using the plurality of acquired front images, andcorrects scanning positions according to the subject's eye movement.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an optical coherencetomographic imaging apparatus includes an image acquisition unitconfigured to acquire a plurality of images of a subject's eye atdifferent times, a tomographic image acquisition unit configured toacquire a plurality of tomographic images of the subject's eye based onan interference light produced by the interference between return lightfrom the subject's eye irradiated with measuring light via a scanningunit, and reference light corresponding to the measuring light, amovement amount acquisition unit configured to acquire the amount ofsubject's eye movement based on the plurality of images, a determinationunit configured to determine whether the amount of subject's eyemovement before a scan by the scanning unit exceeds a threshold value,and a control unit configured to, in a case where the amount ofsubject's eye movement before the scan is equal to or smaller than thethreshold value, control the scanning unit to perform scanning positioncorrection between the scan and the next scan based on the amount ofmovement.

Further features and aspects of the present invention will becomeapparent from the following detailed description of exemplaryembodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate exemplary embodiments, features,and aspects of the invention and, together with the description, serveto explain the principles of the invention.

FIG. 1 illustrates an example of a configuration of an optical coherencetomographic imaging apparatus according to a first exemplary embodiment.

FIG. 2 is a flowchart illustrating an example of automatic alignmentaccording to the first exemplary embodiment.

FIG. 3 is a flowchart illustrating an example of fundus trackingaccording to the first exemplary embodiment.

FIG. 4 illustrates an example of a tomographic image captured undersuitable alignment conditions according to the first exemplaryembodiment.

FIG. 5 illustrates an example of a tomographic image captured when aneye is moving according to the first exemplary embodiment.

FIG. 6 illustrates an example of a tomographic image captured duringexecution of automatic alignment according to the first exemplaryembodiment.

FIG. 7 illustrates examples of a plurality of tomographic imagescaptured during execution of automatic alignment according to the firstexemplary embodiment.

FIG. 8 illustrates an example of a virtual tomographic image generatedfrom a plurality of tomographic images according to the first exemplaryembodiment.

FIG. 9 is a flowchart illustrating an example of automatic alignmentcontrol according to the first exemplary embodiment.

FIG. 10 illustrates an example of a scanning pattern without fundustracking control according to the first exemplary embodiment.

FIG. 11 illustrates an example of a tomographic image acquired throughscanning according to the first exemplary embodiment.

FIG. 12 is a flowchart illustrating an example of fundus trackingcontrol according to the first exemplary embodiment.

FIG. 13 illustrates an example of a scanning pattern in fundus trackingcontrol according to the first exemplary embodiment.

FIGS. 14A and 14B illustrate examples of configurations of an opticalcoherence tomographic imaging apparatus according to a second exemplaryembodiment.

FIG. 15 is a flowchart illustrating an example of fundus trackingcontrol according to a third exemplary embodiment.

FIG. 16 is a flowchart illustrating an example of fundus trackingcontrol according to a fourth exemplary embodiment.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the inventionwill be described in detail below with reference to the drawings.

Generally, there is a time lag from when the subject's eye movement isdetected till when scanning position correction is performed. Therefore,in a case where there arises a comparatively large subject's eyemovement per unit time (such as a flick movement which is one ofinvoluntary eye movements during fixation), a distortion will arise bythis movement in a tomographic image even if this movement is tracked.Further, the distortion will arise also in correcting a scanningposition deviation due to this movement.

One of objectives of the present exemplary embodiment is to acquire atomographic image with reduced distortion due to this movement even ifthere arises a comparatively large subject's eye movement per unit timeduring tracking of the subject's eye movement.

The optical coherence tomographic imaging apparatus according to thepresent exemplary embodiment can acquire the amount of subject's eyemovement (for example, the amount of subject's eye rotation) based on aplurality of images of the subject's eye (for example, a plurality offundus images) acquired at different times. The optical coherencetomographic imaging apparatus according to the present exemplaryembodiment can determine whether the amount of subject's eye movementbefore a scan by a scanning unit exceeds a threshold value. In a casethe amount of subject's eye movement before the scan is equal to orsmaller than the threshold value, the optical coherence tomographicimaging apparatus according to the present exemplary embodiment cancontrol the scanning unit to perform scanning position correctionbetween the scan and the next scan based on the amount of subject's eyemovement.

An optical coherence tomographic imaging apparatus according to anotherexemplary embodiment can control the scanning unit, in a case where theamount of subject's eye movement before the scan exceeds a thresholdvalue, to restart scan of the scanning unit from a scanning positionbefore the scan.

An optical coherence tomographic imaging apparatus according to stillanother exemplary embodiment can detect a blink of the subject's eyebefore a scan by the scanning unit, based on a plurality of images ofthe subject's eye acquired at different times. An optical coherencetomographic imaging apparatus according to still another exemplaryembodiment can control the scanning unit, in a case where a blink of thesubject's eye is detected, to restart scan of the scanning unit from ascanning position before the scan.

An optical coherence tomographic imaging apparatus according to stillanother exemplary embodiment can control the scanning unit (control theoperation of a unit for tracking the subject's eye) to perform scanningposition correction between a scan and the next scan of the scanningunit based on a plurality of images of the subject's eye acquired atdifferent times.

According to at least one of the above-described exemplary embodiments,it is possible to acquire a tomographic image by reducing distortion dueto the relevant subject's movement, even in a case where a comparativelylarge subject's eye movement per unit time arises.

In a case fundus tracking is activated during tomographic imagecapturing, a distortion may arise in a tomographic image due tocorrection of a scanning position through fundus tracking. In a case theinterval of scanning position correction through fundus tracking isshorter than the time for acquiring information in the depth direction(A scan acquisition time) at one point on the subject's eye, nodistortion arises in the tomographic image because the scanning positioncorrection is suitably performed at each scanning point for acquiringone tomographic image. However, it is difficult to make the interval ofscanning position correction through fundus tracking shorter than the Ascan acquisition time. For example, in fundus tracking, since a frontfundus image is often used, it is difficult to make the interval ofscanning position correction through fundus tracking shorter than afront image acquisition interval. Generally, the front image acquisitioninterval is about several tens of milliseconds which are longer than theA scan acquisition interval (generally, several tens of microseconds).Therefore, it is difficult to perform scanning position correctionthrough fundus tracking for each point during scan on the subject's eye.Scanning position correction is performed at regular intervals for eachamount of scanning range. After scanning position correction isperformed at regular intervals, eye movements detected at regularintervals will be corrected at one time. As a result, in a case wherescan is performed on the subject's eye, a rapid change in the scanningposition will be produced at regular intervals. Such a rapid change inthe scanning position appears as a tomographic deviation (distortion) atregular intervals on a captured tomographic image.

Such a tomographic image distortion not only disturbs image diagnosis bya doctor but also causes misrecognition of the tomographic imagedistortion as a lesioned portion, possibly leading to misdiagnosis.Further, a tomographic image distortion may also have an adverse effecton an automatic retina layer boundary recognition function provided inmany optical coherence tomographic imaging apparatuses. If a retinalayer boundary is misrecognized, measurement values of the retina layerthickness based on the misrecognition will be displayed, possiblyleading to misdiagnosis.

To avoid this problem, it is desirable to control the method fortracking the relevant subject's eye in a case where a plurality oftomographic images of the subject's eye is acquired. This enablesacquiring tomographic images with reduced distortion due to thesubject's eye movement. For example, it is desirable to activate themeans for tracking the subject's eye so as to correct the acquisitionposition of the next tomographic image between the time when one of aplurality of tomographic images is acquired and the time when the nexttomographic image is acquired.

In a case where automatic alignment for automatically adjusting therelative positional relation between the subject's eye and an opticalstorage unit is activated during capturing of a tomographic image, asimilar distortion may arise in the tomographic image. In this case, theeccentricity of the image capturing optical axis due to automaticalignment may incline or vertically move the retina on the tomographicimage. In particular, in a case where performing scan a plurality oftimes to acquire a plurality of tomographic images, there arises asituation where the retina is horizontally located in a certaintomographic image, however, inclined in another tomographic image. In acase a plurality of tomographic images having different inclinations iscaptured in this way, a difference in inclination between tomographicimages appears as a retina shape distortion on a three-dimensional imagegenerated from the plurality of tomographic images.

The above-described factors will specifically be described in thefollowing exemplary embodiments.

A first exemplary embodiment will be described below with reference tothe accompanying drawings.

(Overall Configuration of Optical Coherence Tomographic ImagingApparatus)

The following describes an overall configuration of an optical coherencetomographic imaging apparatus according to a first exemplary embodiment,with reference to FIG. 1. The optical coherence tomographic imagingapparatus according to the present exemplary embodiment acquires atomographic image of the subject's eye based on an interference lightproduced by the interference between return light from the subject'seye, irradiated with measuring light via a scanning unit, and referencelight corresponding to the measuring light. The optical coherencetomographic imaging apparatus includes an optical head unit 100, aspectroscope 200, and a control unit 300. The following describes theconfigurations of the optical head unit 100, the spectroscope 200, andthe control unit 300 in this order.

(Configurations of Optical Head Unit 100 and Spectroscope 200)

The optical head unit 100 includes a measuring light optical system forcapturing a two-dimensional image and a tomographic image of an anteriorocular segment Ea and a fundus Er of a subject's eye E. The followingdescribes the inside of the optical head unit 100. An objective lens101-1 is disposed to face the subject's eye E. The optical path isbranched by a first dichroic mirror 102 and a second dichroic mirror 103disposed on the optical axis to function as an optical path separationunit. Specifically, the optical path is branched for each wavelengthband into a measurement path L1 of an OCT optical system, a fundusobservation optical path and a fixation lamp optical path L2, and ananterior ocular segment observation optical path L3.

The optical path L2 is further branched for each wavelength band by athird dichroic mirror 118 into an optical path to an avalanchephotodiode (APD) 115 for fundus observation, and an optical path to afixation lamp 116. Lenses 101-2, 111, and 112 are disposed on theoptical path L2. The lens 111 is driven by a motor (not illustrated) forfocusing adjustment for the fixation lamp and fundus observation. TheAPD 115 has sensitivity in the vicinity of the wavelength ofillumination light for fundus observation (not illustrated),specifically, 780 nanometers. The fixation lamp 116 generates visiblelight to prompt the subject to perform the fixation.

An X scanner 117-1 (for the main scanning direction) and a Y scanner117-2 (for the sub scanning direction intersecting with the mainscanning direction) for scanning the fundus Er of the subject's eye Ewith light emitted from an illumination light source for fundusobservation (not illustrated) are disposed on the optical path L2. Thelens 101-2 is disposed so that its focal position comes to the vicinityof the center position between the X scanner 117-1 and the Y scanner117-2. Although the X scanner 117-1 is a resonance type mirror, it maybe a polygon mirror. The vicinity of the center position between the Xscanner 117-1 and the Y scanner 117-2 has an optically conjugaterelation with the pupillary position of the subject's eye E. The APD 115(single detector) detects light dispersed and reflected by the fundusEr, as return light. The third dichroic mirror 118, which is a prismcomposed of a perforated mirror or a vapor-deposited hollow mirror,separates incident light into the illumination light and the returnlight from the fundus Er.

A lens 141 and an infrared charge-coupled device (CCD) 142 for anteriorocular segment observation are disposed on the optical path L3. Theinfrared CCD 142 has sensitivity in the vicinity of the wavelength ofillumination light for anterior ocular segment observation (notillustrated), specifically, 970 nanometers. The optical path L1 formsthe OCT optical system, as described above, and is used to capture atomographic image of the fundus Er of the subject's eye E. Morespecifically, the optical path L1 is used to acquire an interferencesignal for forming a tomographic image.

A lens 101-3, a mirror 121, and an X scanner 122-1 and a Y scanner 122-2as a scanning unit are disposed on the optical path L1 to deflect lighton the fundus Er of the subject's eye E. Further, the X scanner 122-1and the Y scanner 122-2 are disposed so that the center position betweenthe X scanner 122-1 and the Y scanner 122-2 comes to the focal positionof the lens 101-3. Further, the vicinity of the center position betweenthe X scanner 122-1 and the Y scanner 122-2 has an optically conjugaterelation with the pupillary position of the subject's eye E. With thisconfiguration, the optical paths having the scanning unit as an objectpoint become approximately parallel between the lenses 101-1 and 101-3.This enables providing an identical incident angle for the firstdichroic mirror 102 and the second dichroic mirror 103 even when the Xscanner 122-1 and the Y scanner 122-2 perform scanning.

A measuring light source 130 serves as a light source for emitting themeasuring light into the measurement path. According to the presentexemplary embodiment, the measuring light source 130 is a fiber end, andhas an optically conjugate relation with the fundus Er of the subject'seye E. Lenses 123 and 124 are disposed on the optical path L1. The lens123 is driven by a motor (not illustrated) to perform focusingadjustment. Focusing adjustment is performed so that the light emittedfrom the measuring light source 130 (fiber end) is focused on the fundusEr. The lens 123 which functions as a focusing adjustment unit isdisposed between the measuring light source 130 and the X scanner 122-1and the Y scanner 122-2 as a scanning unit. This makes it unnecessary tomove the larger lens 101-3 and an optical fiber 125-2.

This focusing adjustment enables forming of an image of the measuringlight source 130 on the fundus Er of the subject's eye E, and the returnlight can be efficiently returned from the fundus Er of the subject'seye E to the optical fiber 125-2 via the measuring light source 130.

Referring to FIG. 1, although the optical path between the X scanner122-1 and the Y scanner 122-2 is formed within the drawing paper, it isactually formed in the direction perpendicular to the drawing paper. Theoptical head unit 100 further includes a head driving unit 140. The headdriving unit 140 includes three motors (not illustrated) to enablemoving of the optical head unit 100 in the three-dimensional (X, Y, Z)directions with respect to the subject's eye E. Thus, the optical headunit 100 can be aligned with respect to the subject's eye E.

The following describes the configurations of the optical path from themeasuring light source 130, a reference light optical system, and thespectroscope 200. The measuring light source 130, an optical coupler125, optical fibers 125-1 to 125-4, a lens 151, a dispersioncompensation glass 152, a mirror 153, and a spectroscope 200 form aMichelson interference system. The optical fibers 125-1 to 125-4 (singlemode optical fibers) are connected to and integrated with the opticalcoupler 125.

The light emitted from the measuring light source 130 advances throughthe optical fiber 125-1, and is divided into measuring light on the sideof the optical fiber 125-2 and reference light on the side of theoptical fiber 125-3 by the optical coupler 125. The measuring lightadvances through the above-described optical path of the OCT opticalsystem. The fundus Er of the subject's eye E (observation target) isirradiated with the measuring light. Then, after the reflection anddispersion on the retina, the measuring light reaches the opticalcoupler 125 via the same optical path.

On the other hand, the reference light advances through the opticalfiber 125-3, the lens 151, the dispersion compensation glass 152provided to join dispersion of the measuring light and the referencelight, reaches the mirror 153, and reflects off the mirror 153. Then,the reference light advances through the same optical path, and reachesthe optical coupler 125. The optical coupler 125 combines the measuringlight and the reference light into interference light. In this case,interference occurs when the optical path length for the measuring lightbecomes almost identical to the optical path length for the referencelight. The position of the mirror 153 is adjustably held in the opticalaxis direction by a motor and a drive mechanism (not illustrated),enabling adjusting of the optical path length of the reference light tothe optical path length of the measuring light which changes accordingto the subject's eye E. The interference light is led to thespectroscope 200 via the optical fiber 125-4.

The spectroscope 200 includes a lens 201, a diffraction grating 202, alens 203, and a line sensor 204. The interference light emitted from theoptical fiber 125-4 is converted into approximately parallel light bythe lens 201, subjected to spectral diffraction by the diffractiongrating 202, and then focused on the line sensor 204 by the lens 203.

The following describes the periphery of the measuring light source 130.The measuring light source 130, a typical low-coherent light source, isa super luminescent diode (SLD) having a center wavelength of 855nanometers and a wavelength bandwidth of about 100 nanometers. Thebandwidth affects the resolution of an acquired tomographic image in theoptical axis direction, and therefore serves as an important parameter.Further, although a SLD is selected as a light source, the light sourcetype is not limited thereto and may be, for example, an amplifiedspontaneous emission (ASE) as long as low-coherent light can be emitted.In consideration of ocular measurement, the near-infrared light issuitable for the center wavelength. Since the center wavelength affectsthe resolution of an acquired tomographic image in the horizontaldirection, the center wavelength is desirably as short as possible. Forboth reasons, the center wavelength was set to 855 nanometers.

Although, in the present exemplary embodiment, a Michelsoninterferometer is used as an interferometer, a Mach-Zehnderinterferometer may also be used. In a case the light volume differencebetween the measuring light and the reference light is large, the use ofa Mach-Zehnder interferometer is desirable. In a case a light volumedifference is comparatively small, the use a Michelson interferometer isdesirable.

(Configuration of Control Unit 300)

The control unit 300 is connected with the optical head unit 100 andeach part of the spectroscope 200. Specifically, the control unit 300 isconnected with the infrared CCD 142 in the optical head unit 100 toenable generating an observation image of the anterior ocular segment Eaof the subject's eye E. The control unit 300 is also connected with theAPD 115 in the optical head unit 100 to enable generating a fundusobservation image of the fundus Er of the subject's eye E. Further, thecontrol unit 300 is also connected with the head driving unit 140 in theoptical head unit 100 to enable three-dimensionally driving the opticalhead unit 100 with respect to the subject's eye E.

On the other hand, the control unit 300 is connected also with the linesensor 204 of the spectroscope 200. The spectroscope 200 enablesacquiring measurement signals for respective wavelengths, and furthergenerating a tomographic image of the subject's eye E based on thesemeasurement signals.

The generated anterior ocular segment observation image, fundusobservation image, and tomographic image of the subject's eye E can bedisplayed on a monitor 301 connected to the control unit 300.

(Subject's Eye E Alignment Method)

The following describes a subject's eye E alignment method using theoptical coherence tomographic imaging apparatus according to the presentexemplary embodiment, with reference to the flowchart illustrated inFIG. 2. Prior to image capturing, an inspector makes a subject sit downin front of the apparatus.

In step S201, upon reception of a switch operation (not illustrated) bythe inspector, the control unit 300 starts automatic alignment. In stepS202, the control unit 300 functions as an anterior ocular segment imageacquisition unit. When automatic alignment is started, the control unit300 periodically acquires an anterior ocular segment image from theinfrared CCD 142, and analyzes it. Specifically, the control unit 300detects a pupillary region in the input anterior ocular segment image.

In step S203, the control unit 300 calculates the center position of thedetected pupillary region. In step S204, the control unit 300 functionsas a displacement amount calculation unit, and calculates the amount ofdisplacement between the center position of the detected pupillaryregion and the center position of the anterior ocular segment image. Theoptical coherence tomographic imaging apparatus according to the presentexemplary embodiment is configured so that the center of the anteriorocular segment image coincides with the optical axis of the objectivelens 101-1. The amount of displacement calculated in step S204represents the amount of displacement between the subject's eye E andthe measuring light axis.

In step S205, the control unit 300 instructs the head driving unit 140to move the optical head unit 100 according to the amount ofdisplacement calculated in step S204. In step S206, the head drivingunit 140 drives three motors (not illustrated) to move the position ofthe optical head unit 100 in the three-dimensional (X, Y, Z) directionswith respect to the subject's eye E. As a result of this movement, theposition of the optical axis of the objective lens 101-1 mounted on theoptical head unit 100 is corrected so as to come close to the pupillarycenter position of the anterior ocular segment Ea of the subject's eyeE.

In step S207, after the movement of the optical head unit 100, thecontrol unit 300 determines whether a new anterior ocular segment imageis input from the infrared CCD 142. In a case a new anterior ocularsegment image is determined to have been input (YES in step S207), theprocessing returns to step S202. On the other hand, in a case where anew anterior ocular segment image is determined to have not been input(NO in step S207), the processing exits this flowchart.

With this series of automatic alignments, the optical axis position ofthe objective lens 101-1 constantly moves so as to constantly track thepupillary center position of the anterior ocular segment Ea of thesubject's eye E. Even if the direction of the line of sight of thesubject's eye E changes, this automatic alignment enables the opticalaxis of the objective lens 101-1 to track the pupillary center of theanterior ocular segment Ea after the line of sight is changed (anteriorocular segment tracking). Therefore, the fundus Er is irradiated withthe measuring light emitted from the measuring light source 130 withoutbeing interrupted by the pupil, achieving stable tomographic imagecapturing.

The control unit 300 continues this series of automatic alignments untildeflection of the measuring light on the fundus Er of the subject's eyeE is started to record a tomographic image of the fundus Er of thesubject's eye E.

Although, in the present exemplary embodiment, the control unit 300performs automatic alignment of the optical system for the subject's eyeE based on an anterior ocular segment image captured by the infrared CCD142, automatic alignment may be performed by using other techniques. Forexample, automatic alignment in the three-dimensional (X, Y, Z)directions can be performed by projecting an alignment index onto theanterior ocular segment of the subject's eye E and detecting thereflected light.

(Fundus Tracking Method)

The following describes a fundus tracking method for correcting thedeviation of the measuring light irradiation position due to thesubject's eye E movement when irradiating the fundus Er of the subject'seye E with the measuring light to observe the state of the subject's eyeE, with reference to the flowchart illustrated in FIG. 3.

In step S301, after the above-described automatic alignment is started,the control unit 300 starts the operation for acquiring atwo-dimensional observation image of the fundus Er which has passedthrough the optical path L2. Specifically, the control unit 300 startsacquiring the reflected light from the fundus Er input from the APD 115.The reflected light from the fundus Er is two-dimensionally andcontinuously deflected on the fundus Er by the X scanner 117-1 and the Yscanner 117-2. Therefore, periodically combining the reflected lightinput from the APD 115 enables periodically acquiring a fundusobservation image of the fundus Er.

In step S302, the control unit 300 starts fundus tracking based on theperiodically acquired fundus observation image. In step S303, thecontrol unit 300 calculates the amount of fundus Er movement by usingtwo fundus observation images (a previously acquired fundus observationimage and the current fundus observation image). Specifically, thecontrol unit 300 calculates the amount of displacement between targetregions on the fundus observation images in the two-dimensional (X, Y)directions to calculate the amount of fundus Er movement in thetwo-dimensional (X, Y) directions. The control unit 300 is an example ofa movement amount acquisition unit for acquiring the subject's eyemovement based on a plurality of subject's eye E images (for example, aplurality of fundus images) acquired at different times. Further, atarget region is the macula of the fundus Er, the optic disc, a bloodvessel branch, etc., and may be any desired region on the fundus Er aslong as the amount of fundus Er movement can be calculated.

In step S304, according to the calculated amount of fundus Er movement,the control unit 300 controls the X scanner 122-1 and the Y scanner122-2 to perform scanning position correction so that an identicalregion on the fundus Er is constantly irradiated with the measuringlight that takes the optical path L1.

In step 305, the control unit 300 determines whether a newtwo-dimensional observation image of the fundus Er has been acquired. Ina case the new two-dimensional observation image of the fundus Er isdetermined to have been acquired (YES in step S305), the processingreturns to step S303. On the other hand, in a case the newtwo-dimensional observation image of the fundus Er is determined to havenot been acquired (NO in step S305), the processing exits thisflowchart.

With this series of fundus tracking, the measuring light radiated fromthe measuring light source 130 onto the fundus Er moves so as toconstantly track the movement of the fundus Er of the subject's eye E,achieving stable tomographic image capturing. The control unit 300continues this series of fundus tracking until the inspection of thesubject's eye E is completed.

Although, in the present exemplary embodiment, the control unit 300performs fundus tracking by using fundus observation images based on aspot scanning laser ophthalmoscope (SLO), fundus tracking may beperformed by using other techniques. For example, the control unit 300can perform fundus tracking by using two-dimensional fundus observationimages acquired through the combination of the infrared light which canbe broadly radiated onto the fundus and an infrared CCD. Fundus trackingcan also be performed by projecting any desired pattern formed by alight source onto the fundus Er, and detecting the reflected light.

(Tomographic Image Capturing Method)

The following describes a tomographic image capturing method using theoptical coherence tomographic imaging apparatus according to the presentexemplary embodiment.

The inspector operates a switch (not illustrated) on the control unit300 to start image capturing. In response to an instruction for startingimage capturing, the control unit 300 starts the generation of atomographic image which is to be recorded, based on the interferencelight periodically output from the line sensor 204.

The interference light output from the line sensor 204 is a signal foreach frequency subjected to spectral diffraction by the diffractiongrating 202. The control unit 300 performs the fast Fourier transform(FFT) processing on the signal of the line sensor 204 to generateinformation in the depth direction at a certain point on the fundus Er.The generation of information in the depth direction at the certainpoint on this fundus Er is referred to as A scan.

The control unit 300 drives at least either one of the X scanner 122-1and the Y scanner 122-2 to irradiate the fundus Er with the measuringlight, thus the fundus Er can be arbitrarily scanned. The X scanner122-1 and the Y scanner 122-2 enable deflecting the measuring light onthe subject's eye E for scanning.

The control unit 300 combines a plurality of A scans acquired during ascan on an arbitrary locus into a two-dimensional image to generate atomographic image of the fundus Er on an arbitrary locus.

Further, the control unit 300 drives at least either one of the Xscanner 122-1 and the Y scanner 122-2 to repeat the above-described scanon an arbitrary locus a plurality of times. Performing the same locusoperation a plurality of times enables acquiring a plurality oftomographic images on an arbitrary locus on the fundus Er. For example,the control unit 300 drives only the X scanner 122-1 to repetitivelyperform scan in the X direction to generate a plurality of tomographicimages on the same scanning line of the fundus Er. Further, the controlunit 300 can simultaneously drive the X scanner 122-1 and the Y scanner122-2 to repetitively perform a circular operation to generate aplurality of tomographic images on an identical circle of the fundus Er.The control unit 300 performs the addition average on the plurality oftomographic images to generate one high-definition tomographic image,and displays it on the monitor 301.

On the other hand, the control unit 300 can drive at least either one ofthe X scanner 122-1 and the Y scanner 122-2 to perform scanning aplurality of times while shifting each scan on an arbitrary locus in theX and Y directions. For example, the control unit 300 performs scanningin the X direction a plurality of times while shifting each scan atregular intervals in the Y direction to generate a plurality oftomographic images covering the entire rectangular region on the fundusEr. Then, the control unit 300 combines the plurality of tomographicimages to generate three-dimensional information of the fundus Er, anddisplays the information on the monitor 301.

These scanning patterns by the X scanner 122-1 and the Y scanner 122-2can be arbitrarily switched by pressing a scanning pattern selectionbutton (not illustrated).

(Automatic Alignment Control During Tomographic Image Capturing)

In a case performing scanning a plurality of times as described above tocapture a plurality of tomographic images, the time required to performscanning a plurality of times is longer than the time required toperform a scanning. For example, in the optical coherence tomographicimaging apparatus according to the present exemplary embodiment, thecontrol unit 300 is able to repeat 128 times a 10-mm scan in the Xdirection on the fundus Er while shifting each scan by 0.078 millimetersin the Y direction. These 128 scans enable acquiring 128 tomographicimages and generating three-dimensional information for a 10 mm×10 mmrange on the fundus Er. In the optical coherence tomographic imagingapparatus according to the present exemplary embodiment, one tomographicimage is composed of a total of 1024 A scans. Each A scan takes 14.3microseconds. Therefore, since the acquisition of one tomographic imagetakes 1024×14.3 microseconds=14.6 milliseconds, the acquisition of allof the 128 tomographic images takes at least 14.6 milliseconds×128=1.87seconds.

Human eye movements can be roughly classified into three differenttypes: saccade, drift, and tremolo. These eye movements are kinds ofinvoluntary movement, and are difficult to completely stop even if thesubject gazes at a fixation lamp. The generation interval of these eyemovements is shorter than the above-described image capturing interval(1.87 seconds). In many cases, these eye movements occur a plurality oftimes during execution of all of 128 scans.

However, changes in the pupillary position due to these eye movements donot largely affect captured tomographic images. FIG. 4 illustrate anexample of a tomographic image captured in a state where the pupillarycenter of the anterior ocular segment Ea of the subject's eye Ecoincides with the optical axis of the objective lens 101-1. On theother hand, FIG. 5 illustrate an example of a tomographic image capturedin a state where the pupillary center is deviated by about 1 millimeterin the X direction with respect to the optical axis of the objectivelens 101-1. In the tomographic image of the fundus Er illustrated inFIG. 5, a retina R is deviated in the X direction in comparison with thetomographic image illustrated in FIG. 4. However, the tomographic imageitself has not largely changed in shape. This kind of deviation in the Xdirection can be corrected by the above-described fundus tracking.

On the other hand, in a case automatic alignment is activatedaccompanying the eye movement, the eye movement largely affects acaptured tomographic image. FIG. 6 illustrates an example of atomographic image captured in the state illustrated in FIG. 5 exceptthat automatic alignment is activated so that the pupillary centercoincides with the optical axis of the objective lens 101-1. Incomparison with the tomographic image illustrated in FIG. 4, a deviationin the X direction arises and further the retina R is largely inclined.This kind of inclination of the retina R cannot be corrected by fundustracking. In a case automatic alignment is activated while all of 128scans are being carried out, the inclination of the retina R willlargely change in the middle of acquiring 128 tomographic images, asillustrated in FIG. 7. Such changes in the inclination of the retina Rproduce a noticeable problem particularly in a three-dimensional imagewhich is generated by reconfiguring a plurality of tomographic images.FIG. 8 illustrates an example display of a virtual tomographic imageperpendicularly intersecting with the main scanning direction, generatedby reconfiguring the 128 tomographic images illustrated in FIG. 7. Inthis virtual tomographic image, the retina R has largely changed inshape. For an ophthalmologist who diagnoses eye disease based on thecondition of the retina R, changes in the inclination of the retina Rnot only disturb diagnosis but also lead to misdiagnosis.

Therefore, the optical coherence tomographic imaging apparatus accordingto the present exemplary embodiment suspends automatic alignment duringexecution of scans for capturing a plurality of tomographic images. Thisoperation will be described below with reference to the flowchartillustrated in FIG. 9. Prior to image capturing, the inspector makes thesubject sit down in front of the apparatus. The control unit 300 drivesat least either one of the X scanner 122-1 and the Y scanner 122-2 as ascanning unit to switch between observation scan for capturing atomographic image (hereinafter referred to as a tomographic image forobservation) for subject's eye observation and recording scan forcapturing a tomographic image (hereinafter referred to as a tomographicimage for recording) for subject's eye state recording.

In step S901, upon reception of a switch operation (not illustrated) bythe inspector, the control unit 300 starts automatic alignment. In stepS902, to observe alignment conditions, the control unit 300 startscapturing an observation tomographic image of the fundus Er.

In step S903, the control unit 300 displays the acquired observationtomographic image on the monitor 301. The inspector can determine rightand wrong of alignment conditions with reference to the observationtomographic image displayed on the monitor 301. In a case the inspectordetermines that alignment conditions are right, the inspector operates aswitch (not illustrated) on the control unit 300 to instruct to starttomographic image capturing.

In step S904, in response to a switch operation (not illustrated) by theinspector, the control unit 300 starts capturing tomographic images forrecording. In step S905, upon reception of an image capturing startinstruction, the control unit 300 suspends automatic alignment beforestarting image capturing for recording.

In step S906, the control unit 300 starts scanning for generating aplurality of tomographic images for recording. Specifically, the controlunit 300 controls at least either one of the X scanner 122-1 and the Yscanner 122-2 to perform scanning on an arbitrary locus a plurality oftimes.

In step S907, upon completion of all scans, the control unit 300restarts automatic alignment. In step S908, the control unit 300generates a plurality of tomographic images corresponding to theplurality of scans. In step S909, the control unit 300 records theplurality of tomographic images generated in step S908 in a recordingmedium (not illustrated). This completes the processing of the flowchartillustrated in FIG. 9.

Although, in the present exemplary embodiment, automatic alignment issuspended immediately before starting scans for acquiring tomographicimages for recording, automatic alignment may be suspended before thattiming. Specifically, automatic alignment may be suspended when thepupillary position of the subject's eye E is determined to almostcoincide with the optical axis of the optical system through automaticalignment.

Further, a reception unit for receiving a signal for acquiring aplurality of tomographic images may be provided, and the processing fortomographic image acquisition may be started after reception of thesignal.

As described above, in the optical coherence tomographic imagingapparatus according to the present exemplary embodiment, the controlunit 300 suspends automatic alignment at least when generatingtomographic images for recording, enabling acquiring suitabletomographic images having less distortion.

(Fundus Tracking Control During Tomographic Image Capturing)

Also in a case fundus tracking is performed during scan for acquiringone tomographic image, the captured tomographic image is largelyaffected. As described above, in the optical coherence tomographicimaging apparatus according to the present exemplary embodiment, theacquisition of one tomographic image takes 14.6 milliseconds. Therefore,in a case acquiring a plurality of tomographic images, the control unit300 scans the fundus Er a plurality of times with a period of about 14.6milliseconds. This interval depends on the number of A scans required toform one tomographic image, and on the time required to acquire one Ascan. On the other hand, in the optical coherence tomographic imagingapparatus according to the present exemplary embodiment, the period ofscanning position correction through fundus tracking is 33.3milliseconds. This period depends on the acquisition interval of fundusobservation images of the fundus Er which is used to calculate theamount of displacement for scanning position correction.

Thus, in a case the acquisition interval of tomographic images differsfrom the acquisition interval of fundus observation images, the controlunit 300 performs scanning position correction at timings Ci (i=1 to 3)through fundus tracking during the scan of the fundus Er to acquire onetomographic image, as illustrated in FIG. 10. Although, in fundustracking, the scanning position correction is performed at longintervals, actual correction is performed at very high speed. Therefore,the operation of fundus tracking is performed in such a way thatscanning position correction is instantaneously carried out in responseto all of eye movements occurring within the correction intervals.Therefore, in a case scanning position correction through fundustracking is performed during the scan on the fundus Er to acquire onetomographic image, a gap G of the retina layer will appear, asillustrated in FIG. 11. For an ophthalmologist who diagnoses eye diseasebased on the shape of the retina layer, the gap G of the retina layernot only disturbs diagnosis but also leads to misdiagnosis.

In the optical coherence tomographic imaging apparatus according to thepresent exemplary embodiment, when capturing a plurality of tomographicimages, the control unit 300 performs scanning position correctionthrough fundus tracking between scans for each tomographic imageacquisition, and suspends the scanning position correction during scan.This operation will be described below with reference to the flowchartillustrated in FIG. 12. Prior to image capturing, the inspector makesthe subject sit down in front of the apparatus. The control unit 300 candrive at least either one of the X scanner 122-1 and the Y scanner 122-2as a scanning unit to switch between observation scan for capturing atomographic image for subject's eye observation and recording scan forcapturing a tomographic image for subject's eye state recording.

In step S1201, upon reception of a switch operation (not illustrated) bythe inspector, the control unit 300 starts automatic alignment. Then, toobserve alignment conditions, the control unit 300 starts capturing anobservation tomographic image of the fundus Er. In step S1202, thecontrol unit 300 displays the acquired observation tomographic image onthe monitor 301. The inspector can determine right or wrong of alignmentconditions with reference to the observation tomographic image displayedon the monitor 301.

In step S1203, the inspector determines that alignment conditions areright, and, upon reception of a switch operation (not illustrated), thecontrol unit 300 starts capturing tomographic images for recording. Insteps S1201 to S1203, to adjust a coherence gate, the control unit 300may perform scanning position correction based on fundus tracking.

In step S1204, the control unit 300 drives at least either one of the Xscanner 122-1 and the Y scanner 122-2 as a scanning unit to start a scanon an arbitrary locus.

In step S1205, the control unit 300 functions as a fundus imageacquisition unit, and determines whether a captured fundus image hasbeen acquired. In a case it is determined that a fundus image has beenacquired (YES in step S1205), the processing proceeds to step S1206. Onthe other hand, in a case it is determined that a fundus image has notbeen acquired (NO in step S1205), the processing proceeds to step S1208.

In step S1206, the control unit 300 functions as a movement amountcalculation unit, and calculates the amount of fundus Er movement basedon fundus images already acquired and newly acquired.

In step S1207, the control unit 300 stores in memory (not illustrated)information indicating that the fundus Er movement was detected during ascan, and information indicating the amount of the detected fundus Ermovement. Then, the processing proceeds to step S1208. In step S1208,the control unit 300 ends a scan.

In step S1209, based on the information stored in memory (notillustrated), the control unit 300 determines whether the fundus Ermovement has been detected during execution of a scan. In a case it isdetermined that the fundus Er movement has been detected (YES in stepS1209), the processing proceeds to step S1210. On the other hand, in acase it is determined that the fundus Er movement has not been detected(NO in step S1209), the processing proceeds to step S1212.

In step S1210, the control unit 300 reads the calculated amount offundus Er movement from memory (not illustrated). In step S1211, thecontrol unit 300 calculates the next scanning starting positioncorrected by offsetting the amount of fundus Er movement, and moves thenext scanning position to the offset scanning starting position.

In step S1212, the control unit 300 drives at least either one of the Xscanner 122-1 and the Y scanner 122-2 as a scanning unit to move thescanning position to the next scanning starting position.

In step S1213, the control unit 300 determines whether a series of scansis completed. In a case a series of scans is determined to be completed(YES in step S1213), the processing proceeds to step S1214. On the otherhand, in a case it is determined that the next scan has not beenperformed (NO in step S1213), the processing returns to step S1204. Instep S1204, the control unit 300 repeats a series of fundus tracking.

In step S1214, the control unit 300 generates a plurality of tomographicimages for recording corresponding to a series of a plurality of scans.In step S1215, the control unit 300 displays the plurality oftomographic images for recording generated in step S1214 on the monitor301. This completes the processing of the flowchart illustrated in FIG.12. Thus, the control unit 300 suspends the scanning position correctionduring a scan, and performs scanning position correction between a scanand the next scan. The control unit 300 may control the scanning unit toperform the scanning position correction during sub scan by the scanningunit, and to suspend the scanning position correction during main scanby the scanning unit.

The following describes an example of scanning in which the control unit300 scans the fundus Er a plurality of times while performing fundustracking according to the flowchart illustrated in FIG. 12, withreference to FIG. 13. The fundus Er movement is detected at timings Di(i=1 to 3), and scanning position correction is performed based on thecalculated amount of movement at timings Ci (i=1 to 3). As illustratedin FIG. 13, the scanning position correction accompanying the fundus Ermovement detected at the timing D1 is delayed till the next scan isstarted at the timing C1. Similarly, the scanning position correctionaccompanying the fundus Er movement detected at timings D2 and D3 isdelayed till timings C2 and C3, respectively. Performing control in thisway enables completely and continuously scanning the fundus Er of thesubject's eye E without interruption. Therefore, it is possible toreduce the possibility that the gap G of the retina layer as illustratedin FIG. 11 appears on a captured tomographic image for recording. As forthe tomographic images acquired by scans at the timings D1, D2, and D3at which the fundus Er movement was detected, the scanning position isnot corrected, and therefore the possibility that the gap G of theretina layer appears is low. However, since the fundus Er moves duringscan, a distortion may possibly arise in the acquired tomographic imageto a certain extent. Therefore, the control unit 300 may removetomographic images acquired by scans at the timings D1, D2, and D3, orperform again the same scans at respective scanning positions to capturetomographic images again. Thus, tomographic images having lessdistortion can be acquired.

In the processing of the flowchart illustrated in FIG. 12, the controlunit 300 may parallelly perform the automatic alignment suspendingprocessing and the automatic alignment restart processing in steps S905and S907, respectively, illustrated in FIG. 9. Specifically, the controlunit 300 may perform the alignment suspending processing in step S905between the processing in step S1203 and the processing in step S1204,and perform the alignment restart processing in step S907 between theprocessing in step S1213 and the processing in step S1214. Thus, thecontrol unit 300 may perform at least either one of the automaticalignment processing illustrated in FIG. 9 and the scanning positioncorrection processing based on fundus tracking illustrated in FIG. 12.

Although, in the present exemplary embodiment, the control unit 300performs control to perform scanning position correction between scans(between a scan and the next scan) when acquiring a tomographic imagefor recording, the control unit 300 may perform similar control alsowhen acquiring a tomographic image for observation. In this case, adistortion of the retina layer can be reduced also in tomographic imagesfor observation. Further, when acquiring a tomographic image forobservation, instead of performing scanning position correction betweenscans (between a scan and the next scan), the control unit 300 mayperform scanning position correction when the fundus Er movement isdetected. Tomographic images for observation are displayed as areal-time observation moving image, and therefore the display period isvery short. Further, tomographic images for observation are not used fordiagnosis, and therefore a distortion of the retina layer is permissibleto a certain extent.

As described above, in the optical coherence tomographic imagingapparatus according to the present exemplary embodiment, duringexecution of a scan, the control unit 300 suspends at least either oneof the alignment of the optical system relative to the subject's eye forcapturing an image of the subject's eye and the scanning positioncorrection through fundus tracking of the subject's eye. Thus,tomographic images having less distortion can be acquired.

(Configuring Fundus Observation Optical System with Apparatus Other thanSLO Optical System)

A second exemplary embodiment will be described below with reference toFIGS. 14A and 14B. FIGS. 14A and 14B illustrate examples ofconfigurations of an optical coherence tomographic imaging apparatusaccording to the second exemplary embodiment. Although the configurationillustrated in FIG. 14A is almost similar to the configurationillustrated in FIG. 1, an optical path L16 is disposed instead of theoptical path L2.

The configuration illustrated in FIG. 14A will be described below. Theconfiguration illustrated in FIG. 14A uses a CCD instead of an APD toacquire fundus observation images. A lens 1601, a perforated mirror1602, lens 1605 and 1606, and a dichroic mirror 1607 are disposed on theoptical path L16 in this order. A lens 1603 and a fundus observationlight source 1604 are disposed on the reflection side of the perforatedmirror 1602. The dichroic mirror 1607 transmits a wavelength in thevicinity of the fundus observation light source 1604, specifically, 780nanometers. A fundus observation CCD 1608 is disposed on the penetrationside, and a fixation lamp light source 116 is disposed on the reflectionside. The lens 1605 is driven by a motor (not illustrated) for focusingadjustment for the fixation lamp and fundus observation. The CCD 1608has sensitivity in the vicinity of the wavelength of the fundusobservation light source 1604.

Fundus images acquired by the CCD 1508 can be handled in a similar wayto fundus images in the first exemplary embodiment. Therefore, thecontrol unit 300 controls fundus tracking, detection of the amount ofmovement, and the scanning position correction in a similar way to thefirst exemplary embodiment. Although, in the present exemplaryembodiment, a CCD is used, a CMOS or other two-dimensional sensors maybe used.

The configuration illustrated in FIG. 14B will be described below. Theconfiguration illustrated in FIG. 14B is similar to the configurationillustrated in FIG. 14A except that a dedicated fundus observation lightsource, a fundus observation sensor, and a related mirror or lens arenot provided. Similar to the first exemplary embodiment, to acquire afundus observation image, a computer 300 performs the FFT processing ona signal of a licenser 204, and performs control to generate informationin the depth direction at a certain point on the fundus Er. In the firstexemplary embodiment, this information in the depth direction is used toacquire a tomogram image. In the present exemplary embodiment, thisinformation in the depth direction is accumulated for use as informationindicating the state of a certain point on the fundus Er. Similar to thecase of tomographic image acquisition in the first exemplary embodiment,the control unit 300 can accumulate the information in the depthdirection at each point within a 10-mm range in the X direction, andrepeats the relevant processing 128 times to acquire the state of eachpoint within a 10 mm×10 mm range on the fundus Er. By converting theinformation into density and luminance, an image indicating the state ofthe fundus within the above-described range can be acquired.

Using this image as a fundus observation image enables scanning positioncorrection between scans in a similar way to the first exemplaryembodiment. However, in the present exemplary embodiment, since thecontrol unit 300 needs to control the X scanner 122-1 and the Y scanner122-2 to acquire a fundus observation image, the control unit 300 cannotperform scan for tomographic image acquisition during fundus observationimage acquisition. The control unit 300 needs to perform fundusobservation image acquisition between each of the scans. On the otherhand, in the present exemplary embodiment, since neither a dedicatedfundus observation light source nor a fundus observation sensor isprovided, there is an advantage that the cost of the apparatus can bereduced.

(Determining Whether Amount of Subject's Eye Movement Exceeds ThresholdValue)

FIG. 15 is a flowchart illustrating an example of fundus trackingcontrol according to a third exemplary embodiment. The control unit 300performs alignment of the subject's eye, fundus tracking, andtomographic image capturing in a similar way to the first exemplaryembodiment.

In the present exemplary embodiment, in a case the amount of subject'seye movement before a scan is equal to or smaller than a thresholdvalue, the control unit 300 performs scanning position correctionbetween a scan and the next scan based on the relevant amount ofmovement. Further, in a case the mount of subject's eye movement beforea scan exceeds the threshold value, the control unit 300 restarts scanof the scanning unit from a scanning position before a scan.

In a case performing scanning position correction between a scan and thenext scan, if the fundus Er movement is very fast or if the processingspeed for scanning is not sufficient, the control unit 300 may notperform scanning position correction in time. Then, a first distortionarises in the tomographic image caused by the fundus Er movement, andthen a second distortion arises due to scanning position correction atdelayed timing. In this way, the scanning position correction mayincrease the number of tomographic image distortions. To solve thisproblem, the control unit 300 restarts scan from a scan before thefundus Er movement occurs. Thus, tomographic images having lessdistortion can be acquired.

FIG. 15 is a flowchart illustrating processing for restarting scan bythe scanning unit. The flowchart illustrated in FIG. 15 is almostidentical to the flowchart illustrated in FIG. 12 except that stepsS1401 and S1402 are added. Specifically, up to step S1211, the controlunit 300 performs scanning position correction in a similar way to thefirst exemplary embodiment. In step S1401, the control unit 300 as anexample of a determination unit determines whether the amount ofsubject's eye movement before a scan by the scanning unit exceeds athreshold value. This amount of movement refers to the amount of fundusEr movement occurring during N scans. The value of N is anapparatus-specific value and is determined by the frame rate for fundusimage acquisition, the frame rate for scan, and the fundus imageprocessing time. In the present exemplary embodiment, N is about 5. Thethreshold value of the amount of fundus Er movement is determined by theresolution of the apparatus between the scans and the frame rate forfundus image acquisition. In the present exemplary embodiment, thethreshold value is about 10 to 100 micrometers. In a case the amount offundus Er movement is determined to be smaller than the threshold value(NO in step S1401), then in step S1212, the control unit 300 performsprocessing in subsequent steps in a similar way to the first exemplaryembodiment. On the other hand, in a case the amount of fundus Ermovement is determined to be equal to or greater than the thresholdvalue (YES in step S1401), then in step S1402, the control unit 300offsets the scanning starting position. The control unit 300 performsthis offsetting of the past N scans in a backward direction. Forexample, in the present exemplary embodiment, the control unit 300repeats 128 times a 10-mm scan in the X direction on the fundus Er whileshifting each scan by 0.078 millimeters in the Y direction. In thiscase, the offset is 0.078×N millimeters in the Y direction. In this way,the control unit 300 achieves control for restarting scan from a pastscan.

Thus, even if the control unit 300 cannot perform scanning positioncorrection in time, the control unit 300 restarts scan from a scanbefore a tomographic image distortion occurs. Thus, tomographic imageshaving less distortion can be acquired.

(Detection of Amount of Fundus Rotation)

A fourth exemplary embodiment will be described below. The apparatusconfiguration according to the present exemplary embodiment is similarto that according to the first exemplary embodiment, and redundantdescriptions thereof will be omitted. The control unit 300 performsalignment of the subject's eye, fundus tracking, and tomographic imagecapturing in a similar way to the first exemplary embodiment.

The present exemplary embodiment differs from the first exemplaryembodiment in that it detects the amount of fundus Er rotation inaddition to the amount of fundus Er movement. In the first exemplaryembodiment, since the control unit 300 detects the amount of fundus Ermovement in the X and Y directions to perform scanning positioncorrection, a tomographic image may not provide a straight line on thefundus Er if the angle of the subject's eye E changes during processing.Then, the control unit 300 detects the amount of fundus rotation tocorrect the rotational direction of scan.

Specifically, the control unit 300 detects the amount of fundus rotationas follows. The control unit 300 sets two target regions on each fundusobservation image, and detects respective target regions in the previousfundus observation image and the current fundus observation image. Thecoordinates of the target regions detected in the previous fundusobservation image are A1 (xa1, ya1) and B1 (xb1, yb1), and thecoordinates of the target regions detected in the current fundusobservation image are A2 (xa2, ya2) and B2 (xb2, yb2). In this case, A2indicates the same target region as A1, and B2 indicates the same targetregion as B1.

Generally, the coordinate transformation based on the combination ofparallel translation, and rotation in the two-dimensional coordinatesare represented by an affine transformation matrix. The control unit 300performs the coordinate transformation from the coordinates of theprevious fundus observation image into the coordinates of the currentfundus observation image as follows. First, the control unit 300performs parallel translation so that the target region A1 coincideswith the origin (0, 0). The control unit 300 sets a vector (tx1, ty1)for representing this translation. Then, the control unit 300 performsrotation centering on the origin (=A1) so that a vector A2B2 (xb2-xa2,yb2-ya2) coincides with a vector A1B1 (xb1-xa1, yb1-ya1). This rotationis performed with a rotational angle θ. Finally, the control unit 300performs translation so that the origin (=A1) coincides with the targetregion A2. The control unit 300 sets a vector (tx2, ty2) forrepresenting this translation. This translation can be represented by anaffine transformation matrix as follows.

$\begin{matrix}{\begin{pmatrix}x^{\prime} \\y^{\prime} \\1\end{pmatrix} = {\begin{pmatrix}1 & 0 & {{tx}\; 2} \\0 & 1 & {{ty}\; 2} \\0 & 0 & 1\end{pmatrix}\begin{pmatrix}{\cos\;\theta} & {{- \sin}\;\theta} & 0 \\{\sin\;\theta} & {\cos\;\theta} & 0 \\0 & 0 & 1\end{pmatrix}\begin{pmatrix}1 & 0 & {{tx}\; 1} \\0 & 1 & {{ty}\; 1} \\0 & 0 & 1\end{pmatrix}\begin{pmatrix}x \\y \\1\end{pmatrix}}} & \left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(x, y) indicates the coordinates before transformation, and (x′, y′)indicates the coordinates to be acquired after transformation. Thecontrol unit 300 performs coordinate transformation for all ofsubsequent scans by using this coordinate transformation matrix.

FIG. 16 illustrates a flowchart to which processing for correcting theamount of fundus rotation is added. Although the flowchart illustratedin FIG. 16 is almost similar to the flowchart illustrated in FIG. 12except that the control unit 300 generates and applies a coordinatetransformation matrix formed through the affine transformation based onthe target regions A and B. After a fundus image has been acquired, instep S1501, the control unit 300 detects the target regions A and B. Instep 1502, the control unit 300 stores the coordinates of the detectedtarget regions in memory. In step S1503, the control unit 300 reads thecoordinates of the target regions A and B from memory. In step S1504,based on the coordinates (A1, B1, A2, B2) of the target regions A and B,the control unit 300 generates a coordinate transformation matrixthrough the affine transformation. In step 1505, the control unit 300transforms the coordinates of subsequent scans by using the generatedcoordinate transformation matrix. Thus, the control unit 300 cancontinue scanning so that the scan locus forms a straight line on thefundus Er.

(Detection of Subject's Eye Blink)

A fifth exemplary embodiment will be described below. The apparatusconfiguration according to the present exemplary embodiment is similarto that according to the second exemplary embodiment, and redundantdescriptions thereof will be omitted. In the present exemplaryembodiment, the control unit 300 detects the amount of fundus Ermovement, detects a blink of the subject's eye, and stores and readscoordinates in/from memory according to the flowchart illustrated inFIG. 15. Further, the condition in step S1401 is changed to “amount offundus movement threshold value, or is a subject's eye blink detected?”.Processing for other control is similar to that according to the thirdexemplary embodiment.

According to the present exemplary embodiment, the control unit 300determines a blink of the subject's eye as follows. First, the controlunit 300 as an example of a detection unit detects the target regionsdescribed in the first exemplary embodiment in a plurality of fundusobservation images acquired in succession. In a case the detection ofthe target region fails once or a plurality of times in succession, andthen the detection of the same target region is successfully completedonce or a plurality of times in succession, the control unit 300determines that the subject's eye Er has blinked. The upper and thelower limits of the number of times of successive success or failure areapparatus-specific values which are determined by comparing the framerate for fundus observation image acquisition with the blinking time ofa healthy eye.

With the above-described control, in a case the subject's eye Er blinks,the control unit 300 can return to a scan before the blink and thenrestart scan. Therefore, even if a blink causes information loss andsuitable tomographic image acquisition fails, the above-describedcontrol enables acquiring of suitable tomographic images withoutinformation loss. In addition to the detection using the target region,the control unit 300 may detects a blink by using anterior ocularsegment images for the subject's eye. For example, the control unit 300may determine the detection of a blink in a case the area of thepupillary region in the anterior ocular segment image is smaller than athreshold value.

Other Embodiments

Embodiments of the present invention can also be realized by a computerof a system or apparatus that reads out and executes computer executableinstructions recorded on a storage medium (e.g., non-transitorycomputer-readable storage medium) to perform the functions of one ormore of the above-described embodiment(s) of the present invention, andby a method performed by the computer of the system or apparatus by, forexample, reading out and executing the computer executable instructionsfrom the storage medium to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or more ofa central processing unit (CPU), micro processing unit (MPU), or othercircuitry, and may include a network of separate computers or separatecomputer processors. The computer executable instructions may beprovided to the computer, for example, from a network or the storagemedium. The storage medium may include, for example, one or more of ahard disk, a random-access memory (RAM), a read only memory (ROM), astorage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-017661 filed Jan. 31, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An optical coherence tomographic imagingapparatus comprising: an image acquisition unit configured to acquire aplurality of images of a subject's eye at different times; a tomographicimage acquisition unit configured to acquire a plurality of tomographicimages of the subject's eye based on interference light obtained byinterfering return light from the subject's eye irradiated withmeasuring light via a scanning unit and reference light corresponding tothe measuring light; a movement amount acquisition unit configured toacquire the amount of subject's eye movement based on the plurality ofimages; a determination unit configured to determine whether the amountof subject's eye movement before a scan by the scanning unit exceeds athreshold value; and a control unit configured to control, in a case theamount of subject's eye movement before the scan is equal to or smallerthan the threshold value, the scanning unit to perform scanning positioncorrection between the scan and the next scan based on the amount ofmovement.
 2. The optical coherence tomographic imaging apparatusaccording to claim 1, wherein, in a case the amount of subject's eyemovement before the scan exceeds the threshold value, the control unitcontrols the scanning unit to restart at least one scan of the scanningunit from a scanning position before the scan.
 3. The optical coherencetomographic imaging apparatus according to claim 1, wherein the imageacquisition unit acquires the plurality of tomographic images of thesubject's eye as the plurality of fundus images, and wherein themovement amount acquisition unit acquires the amount of subject's eyemovement based on the plurality of fundus images.
 4. The opticalcoherence tomographic imaging apparatus according to claim 3, whereinthe image acquisition unit acquires the plurality of fundus images byprocessing the plurality of tomographic images of the subject's eye. 5.The optical coherence tomographic imaging apparatus according to claim1, wherein the movement amount acquisition unit acquires the amount ofsubject's eye rotation as the amount of movement based on the pluralityof images, and wherein the control unit controls the scanning unit toperform scanning position correction between the scan and the next scanbased on the acquired amount of rotation.
 6. The optical coherencetomographic imaging apparatus according to claim 1, further comprising:a detection unit configured to detect a blink of the subject's eyebefore the scan based on the plurality of images, wherein, in a case theblink is detected, the control unit controls the scanning unit torestart at least one scan of the scanning unit from a scanning positionbefore the scan.
 7. The optical coherence tomographic imaging apparatusaccording to claim 1, wherein the control unit controls the scanningunit to perform scanning position correction between one main scan andthe next main scan of the scanning unit, and to suspend the scanningposition correction during main scan of the scanning unit.
 8. Theoptical coherence tomographic imaging apparatus according to claim 1,wherein the control unit controls the scanning unit to perform scanningposition correction during sub scan of the scanning unit, and to suspendthe scanning position correction during main scan of the scanning unit.9. An optical coherence tomographic imaging apparatus comprising: animage acquisition unit configured to acquire a plurality of images ofthe subject's eye at different times; a tomographic image acquisitionunit configured to acquire a plurality of tomographic images of thesubject's eye based on interference light obtained by interfering returnlight from the subject's eye irradiated with measuring light via ascanning unit and reference light corresponding to the measuring light;a movement amount acquisition unit configured to acquire the amount ofsubject's eye movement based on the plurality of images; and a controlunit configured to control, in a case the amount of subject's eyemovement before a scan by the scanning unit exceeds the threshold value,the scanning unit to restart at least one scan of the scanning unit froma scanning position before the scan.
 10. An optical coherencetomographic imaging apparatus comprising: an image acquisition unitconfigured to acquire a plurality of images of the subject's eye atdifferent times; a tomographic image acquisition unit configured toacquire a plurality of tomographic images of the subject's eye based oninterference light obtained by interfering return light from thesubject's eye irradiated with measuring light via a scanning unit andreference light corresponding to the measuring light; a detection unitconfigured to detect a blink of the subject's eye before a scan by thescanning unit based on the plurality of images; and a control unitconfigured to control, in a case the blink is detected, the scanningunit to restart at least one scan of the scanning unit from a scanningposition before the scan.
 11. A method for controlling an opticalcoherence tomographic imaging apparatus for acquiring a plurality oftomographic images of the subject's eye based on interference lightobtained by interfering return light from the subject's eye irradiatedwith measuring light via a scanning unit and reference lightcorresponding to the measuring light, the method comprising: acquiringthe amount of subject's eye movement based on a plurality of images ofthe subject's eye acquired at different times; determining whether theamount of subject's eye movement before a scan by the scanning unitexceeds a threshold value; and controlling, in a case the amount ofsubject's eye movement before the scan is equal to or smaller than thethreshold value, the scanning unit to perform scanning positioncorrection between the scan and the next scan based on the amount ofmovement.
 12. The method for controlling the optical coherencetomographic imaging apparatus according to claim 11, wherein, in thecontrol, in a case the amount of subject's eye movement before the scanexceeds the threshold value, the method controls the scanning unit torestart at least one scan of the scanning unit from a scanning positionbefore the scan.
 13. The method for controlling the optical coherencetomographic imaging apparatus according to claim 11, wherein theplurality of images is a plurality of fundus images of the subject'seye, and wherein, in acquiring the amount of movement, the methodacquires the amount of subject's eye movement based on the plurality offundus images.
 14. The method for controlling the optical coherencetomographic imaging apparatus according to claim 13, wherein the methodacquires the plurality of fundus images by processing the plurality oftomographic images of the subject's eye.
 15. The method for controllingthe optical coherence tomographic imaging apparatus according to claim11, wherein, in acquiring the amount of movement, the method acquiresthe amount of subject's eye rotation as the amount of movement based onthe plurality of images, and wherein, in the control, the methodcontrols the scanning unit to perform scanning position correctionbetween the scan and the next scan based on the acquired amount ofrotation.
 16. The method for controlling the optical coherencetomographic imaging apparatus according to claim 11, the method furthercomprising: detecting a blink of the subject's eye before the scan basedon the plurality of images, wherein, in the control, in a case the blinkis detected, the method controls the scanning unit to restart at leastone scan of the scanning unit from a scanning position before the scan.17. The method for controlling the optical coherence tomographic imagingapparatus according to claim 11, wherein, in the control, the methodcontrols the scanning unit to perform scanning position correctionbetween one main scan and the next main scan of the scanning unit, andto suspend the scanning position correction during main scan of thescanning unit.
 18. The method for controlling the optical coherencetomographic imaging apparatus according to claim 11, in the control, themethod controls the scanning unit to perform scanning positioncorrection during sub scan of the scanning unit, and to suspend thescanning position correction during main scan of the scanning unit. 19.A method for controlling an optical coherence tomographic imagingapparatus for acquiring a plurality of tomographic images of thesubject's eye based on interference light obtained by interfering returnlight from the subject's eye irradiated with measuring light via ascanning unit and reference light corresponding to the measuring light,the method comprising: acquiring the amount of subject's eye movementbased on a plurality of images of the subject's eye acquired atdifferent times; and controlling, in a case the amount of subject's eyemovement before a scan by the scanning unit exceeds the threshold value,the scanning unit to restart at least one scan of the scanning unit froma scanning position before the scan.
 20. A method for controlling anoptical coherence tomographic imaging apparatus for acquiring aplurality of tomographic images of the subject's eye based oninterference light obtained by interfering return light from thesubject's eye irradiated with measuring light via a scanning unit andreference light corresponding to the measuring light, the methodcomprising: detecting a blink of the subject's eye before a scan by thescanning unit based on a plurality of images of the subject's eyeacquired at different times; and controlling, in a case the blink isdetected, the scanning unit to restart at least one scan of the scanningunit from a scanning position before the scan.